1. Field of the Invention
The present invention relates to an improved process for producing a modified polyisoprene rubber having high green strength by reaction of maleic anhydride with synthetic high cis-1,4-polyisoprene rubber [hereinafter referred to as IR].
2. Description of the Prior Art
IR is a rubber which is widely used and is highly versatile. IR possesses a chemical structure analogous to that of natural rubber and also displays excellent processability characteristics, such as superior workability on rolls, low mill shrinkage and good extrudability; further, vulcanized products consisting thereof exhibit physical properties similar to those of natural rubber. However, IR has one serious shortcoming in that it possesses a rather poor tensile strength while in the unvulcanized state, the so-called "green strength", and this latter phenomenon has been deemed responsible for such adverse properties as bagging and deformation of any such unvulcanizate that could and would take place in any calendering or molding step, prior to vulcanization, in the various schemes for the manufacture of the various rubber articles.
To enhance and improve the green strength of IR, it is known in the art that IR can be reacted with maleic anhydride [hereinafter referred to as MAn]. Such reaction can be carried out, for example, (1) by that method of adding MAn to a solution of IR and thence heating said reaction mixture in the presence of a radical initiator, (2) by that method of reacting MAn with IR, in the solid state and in the absence of a solvent, or (3) by that method of conducting the reaction in an aqueous medium, as described in, for example, U.S. Pat. No. 3,644,248 and British Specification No. 1,168,388. Among the aforesaid methods, it has been considered most advantageous to react the IR with the MAn, in solution, because of the ready control over such reaction conditions as temperature and high uniformity of reaction.
It is technically important that the modification of IR, by reaction with MAn, be applicable on an industrial level, and be conducted economically. And for all practical purposes, it is a desideratum that any process for reacting the IR with the MAn be directly connected, combined or associated with that process employed for the production of the IR; the addition of any further process parameters to the reaction sequence should be avoided to as great an extent as possible. Exemplary in this regard, one such method is known whereby the IR formed during the process for the production thereof is first separated, but then is subsequently dissolved in an extraneous solvent to provide the medium for the reaction with the MAn. While it is of course preferred that the reaction between the MAn and the IR be carried out in solution, nevertheless the immediately aforesaid method is economically undesirable from the point of view that the additional parameters of separation and redissolution of the IR are involved. But since IR is obtained in solution form by the solution polymerization of isoprene, marked technical and economical advantages will be attained if the reaction between the MAn and the IR can be effected by adding the MAn directly to the IR solution originating from said solution polymerization of isoprene.
On an industrial level, IR is typically produced by solution polymerization of isoprene in such solvents as an aliphatic hydrocarbon, for example, n-hexane and butane, or an alicyclic hydrocarbon, for example, cyclohexane. The use of aromatic hydrocarbons as solvents for such polymerization does not produce a product of acceptably high molecular weight; hence, same are not employed on an industrial level. Nevertheless, as above mentioned, the reaction between IR and MAn, by adding the MAn to the solution of the IR in an aliphatic hydrocarbon or alicyclic hydrocarbon solvent which originates from the process for the polymerization of the isoprene, is satisfactorily acceptable in the usual industrial operation. However, upon reaction between IR and MAn in an aliphatic or alicyclic hydrocarbon solvent, considerable difficulties have been encountered in that the efficiency of reaction of the MAn is not sufficiently high and, in particular, it has been observed that the efficiency of the reaction of the MAn markedly diminishes as the amount of MAn utilized in the process is increased.